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| dc.contributor.supervisor | Mbaiwa, Foster | |
| dc.contributor.supervisor | Oyetunji, Olayinka | |
| dc.contributor.author | Nyepetsi, Maipelo | |
| dc.date.accessioned | 2025-09-12T14:10:15Z | |
| dc.date.available | 2025-09-12T14:10:15Z | |
| dc.date.issued | 2024-08 | |
| dc.identifier.citation | Nyepetsi, M (I2024). Improving Mixing Properties of Biodiesel Substrates and Their Flow Properties: Molecular Dynamics studies., Botswana, PhD’s thesis, Botswana International University of Science and Technology: Palapye | en_US |
| dc.identifier.uri | https://repository.biust.ac.bw/handle/123456789/671 | |
| dc.description | Thesis (PhD of Chemical and Forensic sciences)---Botswana International University of Science and Technology, 2024 | en_US |
| dc.description.abstract | Biodiesel is created through a transesterification reaction in which excess methanol reacts with triglycerides in the presence of a catalyst. The reaction is rather slow due to the poor miscibility of triglycerides with methanol, which ultimately increases production costs. Once produced, biodiesel has poor cold flow properties and lacks oxidative stability. These poor cold flow properties make it challenging to use FAMEs during winter months as they tend to form gels, affecting both the engine's performance and durability. This thesis sought solutions to two major limitations of biodiesel production processes slow transesterification during production and poor cold flow properties of neat biodiesel. The first section focused on interactions occurring in binary mixtures of triglycerides and commonly used cosolvents. The structural aspects of these binary mixtures are the focus of the section. The second part was on the interactions in the ternary mixtures, focusing on quantifying the miscibility of methanol in triglycerides in the presence of cosolvents using thermodynamic parameters derived from molecular dynamics simulations. Finally, the third section focused on improving flow and oxidative properties by converting biodiesel to hydrocarbons. This study used triolein, a common triglyceride in oils, as a model fatty acid methyl ester. The structural aspects of six triolein and co-solvent mixtures were investigated to understand the interactions between triglycerides and cosolvents better. The cosolvents used in this study were tetrahydrofuran, hexane, gamma valerolactone, cyclopentyl methyl ester, diethyl ether and dioxane. The results of 100 ns simulations at constant temperature and pressure to simulate mixing show that all binary mixtures are largely unmixed in the first 10 ns and fully mixed at 100 ns except for mixtures containing gamma valerolactone and dioxane. Some solvents interacted strongly with the polar part of triolein, while others interacted strongly with the aliphatic part. Solvation indicators also included the radial distribution functions and clustering of the solvents around triolein. Cosolvents were also found to influence the conformation of triolein molecules. Triolein favoured a propeller conformation in the presence of solvents that solubilize it and adopts a trident conformation when there is little or no solubilisation. The results show that the best solvent for solubilizing triolein is tetrahydrofuran, followed by cyclo pentyl methyl ether, diethyl ether, and hexane. The solubility of 1,4-dioxane was found to increase with xviii temperature. A solvent's miscibility with triolein is aided by its ability to interact with both the polar and non-polar parts of the triolein. In the case of ternary mixtures (triolein/methanol/cosolvent), the Flory-Huggins theory of solutions was applied using molecular dynamics derived parameters. The binary interaction parameters (χij), Gibbs Free energy (∆𝐺 𝑚) and chemical potentials of the components (∆μi) were calculated to study the mixing behaviour). Along the phase boundary, the binary interaction parameters between methanol and cosolvent remain relatively constant. The Gibbs free energy of mixing (∆𝐺 𝑚) in this region shows both positive and negative values, indicating the presence of both mixed and unmixed phases. As expected and consistent with experimental data, ∆𝐺 𝑚 for the systems decreases as one moves from the two phase to single phase part of the ternary system. The activities calculated from Flory-Huggins model were in good agreement with the commonly applied UNIFAC model. The conversion of methyl palmitate to hydrocarbons was studied using Reactive Force Field (ReaxFF) molecular dynamics as a model to improve the flow properties of biodiesel. αNiMoO4, β-NiMoO4, and Ni3Mo were used as catalysts. The results show that the reaction is initiated by the cleavage of the O-CH3 bond, which results in the formation of C16H31O2 • and CH3 • radicals. After its formation, the C16H31O2 • radical is then decarboxylated to produce carbon dioxide and C15H31 • radical. Through β-scission decomposition, this radical produces ethene which is one of the major products. The results also show that the reactions are faster in the presence of catalysts in agreement with experiment. With Ni3Mo as a catalyst, there is evidence of the decomposition of CO2 into CO and C2H4 into C2H2 and H2. Overall, the β-NiMoO4 catalyst performs better in terms of the conversion of methyl palmitate and formation of major products than α-NiMoO4 and Ni3Mo. All the catalysed and uncatalysed decarboxylation of methyl palmitate follows first-order kinetics, from which the activation energy (Ea) was determined. The presence of a catalyst reduced the activation of the reaction from 36.89kcal/mol in an uncatalyzed system to 25.66, 19.34, 15.32, 14.40, and 11.69 kcal/mol for the α-NiMoO4 (110), β-NiMoO4(110), α-NiMoO4 (021), β-NiMoO4 (021), and Ni3Mo (101) catalyzed reactions, respectively. The Ni3Mo catalyzed system has the lowest activation energy of 11.69 kcal/mol, which is comparable to experimental results of 10.11kc. | en_US |
| dc.description.sponsorship | Royal Society and Botswana International University of Science and Technology (BIUST) | en_US |
| dc.publisher | Botswana International University of Science and Technology (BIUST) | en_US |
| dc.subject | Transesterification reaction | en_US |
| dc.subject | Triglycerides | en_US |
| dc.subject | Triolein | en_US |
| dc.subject | Ternary mixtures | en_US |
| dc.subject | Reactive Force Field | en_US |
| dc.subject | Carbon dioxide emissions | en_US |
| dc.subject | Photovoltaic materials | en_US |
| dc.subject | Biofuel | en_US |
| dc.title | Improving Mixing Properties of Biodiesel Substrates and Their Flow Properties: Molecular Dynamics studies | en_US |
| dc.description.level | phd | en_US |
| dc.description.accessibility | unrestricted | en_US |
| dc.description.department | cfs | en_US |
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